I am designing a simple inspection device for checking polarization of lenses. Basically, I need to check horizontal position, vertical position and then any polarizing shift +/- 5deg from vertical position.

So, the lens fixture has to be horizontal, loading of the lens must be horizontal. The rotational measuring device is done by hand, with a straight line indicating horizontal and vertical positions against a "printout" of a protractor.

My question is how to calibrate this device? Assuming I fixture everything at 90 degrees relative to each other, is there a need for calibration, or is this sort of a verification - like checking a tape measure?

I am designing a simple inspection device for checking polarization of lenses. Basically, I need to check horizontal position, vertical position and then any polarizing shift +/- 5deg from vertical position.

So, the lens fixture has to be horizontal, loading of the lens must be horizontal. The rotational measuring device is done by hand, with a straight line indicating horizontal and vertical positions against a "printout" of a protractor.

My question is how to calibrate this device? Assuming I fixture everything at 90 degrees relative to each other, is there a need for calibration, or is this sort of a verification - like checking a tape measure?

Since this appears to impact measurement of product quality, I would recommend finding a way to calibrate it; or at least documenting reasoning why, if it does not end up requiring calibration. Here are some details that I would research if I had to determine if or how to calibrate it.

Look at all the variables and how their accuracies impact product quality. I am ignorant of this type of process, but from your posting, maybe review the angular movement - does the absolute accuracy of the angle impact quality, or is it relative. IF it is relative, how do you maintain repeatability of the angle (if you had to build a second fixture. Does the thickness or flatness, size or shape of the lens impact quality? Also, perhaps calibrating the protractor may provide some traceability.

Or is there a "Golden Unit" that could be used to verify the fixture. If you use a "Golden Unit", how is that unit verified to meet specifications. The method used to verify the "Golden Unit" meets specifications may need to have calibrated instruments. Or, is this fixture the way you finally measure/monitor product specifications?

I know this may not answer your question; but as metrology engineer, these are some of the kinds of questions I might pose when assisting a customer in determing when or where to calibrate. Hope it is of some help.

We just had a quick powwow on this and the question of calibration is on everyone's mind. It seems that, as you suggest, the question is less about calibration and more about repeatability. It isn't possible to calibrate the angular markings, the action will be more of a verification: are the meridians on the angular protractor legible, intact, easy to read, etc.

Thanks for you suggestions, if anyone else can chime in I'd appreciate it.

We just had a quick powwow on this and the question of calibration is on everyone's mind. It seems that, as you suggest, the question is less about calibration and more about repeatability. It isn't possible to calibrate the angular markings, the action will be more of a verification: are the meridians on the angular protractor legible, intact, easy to read, etc.

Thanks for you suggestions, if anyone else can chime in I'd appreciate it.

There has been some tossing about of definitions between the term calibration and verification. Yes, you can perform calibration on a protractor. A device does not have to be "adjustable" to be calibrated - adjustment is just an option for a device that is out of calibration. Of course the other option is to remove the gage from the system. Calibration itself is comparison to a standard - 4 to 10 times more accurate - to ensure the device measures to the standard within its specifications. All other functions - readability, damage, etc. are also handy to do at the time you are performing calibration.

Don't let parsing of the terms dilute the fact: compare to a standard that is more accurate and traceable.